In color printing using digital printers, it is common to use test patches for color calibration. The calibration process involves sending an image with pre-specified device signals (i.e., a “test patch”) to the printer, and making spectrophotometric measurements of the test patches with the use of a spectrophotometric scanner. The device and spectrophotometric signals together are used to build or update the calibration tables or other controls associated with the printer.
U.S. Pat. Nos. 6,048,117 and 6,972,867 relate to the problem of calibrating each of a large population of digital printers, using a single input scanner, as would be used, for example, with a color digital copier. The systems described in these patents use bar codes, or other kinds of machine-readable code, on the sheets on which test patches are printed, in order to identify, among other things, the printer that was the source of each test sheet. U.S. Pat. No. 6,883,892 makes a similar teaching.
In a high-speed, production context, it is known to provide detectors and image sensors immediately downstream of a printing apparatus for various purposes, as shown in U.S. Pat. Nos. 5,488,458; 6,324,353; 6,684,035; and 6,987,025.
U.S. Pat. No. 6,639,669 discloses the use of machine-readable “triggers” on sheets bearing images to be tested.
U.S. Pat. No. 5,148,268 gives a description of a photosensitive chip useful as an image sensor.